1. Field of the Invention
The present invention relates to image signal processing apparatus which provides processing to image signals in order to adjust the image contrast in the display of pictures corresponding to image signals.
2. Related Background Art
Traditionally, when the contrast of pictures is adjusted in the display of pictures corresponding to image signals, the magnitude of a gain to the said image signal has been regulated, for example, irrespective of changes in the level of an input image signal. Therefore, once the contrast level is set, a specified gain is provided whether the level of the input image signal is high or low.
FIG. 1 shows the basic structure to conduct the above conventional contrast adjustment.
In FIG. 1, the input image signal A is amplified at the amplifier 1 according to the gain adjusted by the contrast control knob 2, i.e., the contrast level to be given as output image signal B.
Incidentally, in the case of gain adjustment by the conventional method of contrast control, since the image signals are amplified at a specified gain irrespective of the amplitude of the image signal, there occurs such degradation as white saturation and black crush on the braun tube screen which displays pictures corresponding to the image signals. As a result, the optimum range of contrast becomes very narrow, leading to changes in brightness or incoming light to the braun tube or inability to display good pictures unless the contrast is re-adjusted each time the input image signals change.
On the other hand, there exists such relationship as shown in FIG. 2 between the electric current (IK) of the cathode and the voltage (EGK) between the grid and the cathode of the braun tube (CRT) which is generally used to display pictures corresponding to image signals. Also, there is proportional relationship between the cathode current and brightness. The Ec in the Figure indicates a cut-off voltage, below which there is no glow of cathode current on the braun tube. Es is also the maximum permissible driving voltage, above which the cathode current becomes too large and the spot diameter of electron beam grows large resulting in the inability to show minute part of a picture and increase of X ray generated from the braun tube.
For this reason, the voltage applied to the braun tube should fall between Ec and Es. In other words, the range from Ec to Es is the dynamic range of the braun tube, which determines the contrast ratio of the braun tube (the ratio from black to white).
To put it another way, the braun tube should be operated with the amplitude of image signal within this range in order to gain pictures having sufficient contrast.
However, the image signals do not always move over the entire span of this range. Sometimes, the signal is located at a spot close to the black level indicated on (a) of FIG. 3, namely a small level of an image signal. In order to increase the contrast by utilizing the dynamic range of the braun tube, the variable part of the signal, as shown in (b) of FIG. 3, should be amplified and operated. Based on such basic thought a commercially variable method to utilize effectively the actuation range of the braun tube has been developed in which an autopedestal (FIG. 4) is given to lower automatically the level of pictoral signal corresponding to the most black part of the picture down to the pedestral level or the gain of the amplifier for image signals is raised (as in FIG. 3(b)) to utilize the full actuation range of the braun tube.
But if the autopedestal is done in the above example of the conventional method on such picture which is bright and uniform like a single color of white, sufficient brightness is not gained since the color tends to be drawn to the direction of black. At the same time, the amplitude ratio becomes too large giving uncomfortable picture or if the gain of the amplifier is increased, the standard image signal gives higher contrast than the conventional one but entails such drawbacks as EGK exceeding ES from time to time in the case of image signals which contain white peaks. To prevent this, a limiter is sometimes given. However, since the white peak is limited by the limiter, there is a shortcoming that the while peak (white color) is not sufficient.